1. Technical Field
The present invention relates to an electroacoustic transducer for a speaker configured to reproduce sound by vibrating convex surfaces and a microphone configured to pick up sound.
2. Description of the Related Art
A typical dynamic speaker includes a vibration plate or a diaphragm and a voice coil motor which moves the vibration plate back and forth to cause piston motion for producing sound. The typical dynamic speaker functions substantially as a point sound source and exhibits wide directivity at low frequencies but exhibits sharp directivity over a frequency range equal to or higher than a frequency at which the diameter of a bore of the vibration plate is substantially equal to a half-wavelength of the reproduced sound. Thus, a small speaker using a vibration plate having a small bore is used to reproduce sound at high frequencies. This also applies to a dynamic microphone whose operation principle is reverse to that of the dynamic speaker. That is, a small microphone using a vibration plate having a small bore is used to pick up high frequencies with wide directivity.
In a riffell speaker, in contrast, a vibration plate is constituted by a pair of rectangular curved plates, and the directivity is wide at middle and high frequencies. Also, sound produced by the riffell speaker radiates in a widthwise direction of the vibration plate or a horizontal direction along a direction of curve of the vibration plate and hardly radiates in a lengthwise direction of the vibration plate or a vertical direction.
Patent Document 1 (Japanese Patent Application Publication No. 2002-78079) and Patent Document 2 (Japanese Patent Application Publication No. 2007-174233) disclose conventional riffell speakers.
Patent Document 1 discloses a speaker in which a conductor pattern as a voice coil is printed on a central portion of a polymeric resin film, and the central portion is folded and bonded to form a vibration plate which includes first and second curved vibration portions and a planar plate portion having the conductor pattern, the planar plate portion and first and second curved vibration portions being formed integrally with each other. The planar plate portion of the vibration plate is disposed in a magnetic gap formed in a magnetic circuit, and distal edges of the first and second curved vibration portions are secured to a supporter.
Patent Document 2 discloses a speaker in which a central portion of a vibration plate is folded so as to form a recessed portion in which a flat voice coil wound in an oval annular shape is disposed in two magnetic gaps spaced apart from each other in an up and down direction. Also in this speaker, an outer peripheral portion of the vibration plate is secured to an annular frame.